1. Technical Field
The present invention relates to a liquid ejecting apparatus such as an ink jet printer and a method of controlling the liquid ejecting apparatus, and in particular, to a liquid ejecting apparatus capable of controlling ejection of a liquid by applying a driving pulse contained in a driving signal to a pressure generating element and a method of controlling the liquid ejecting apparatus.
2. Related Art
A liquid ejecting apparatus is an apparatus which includes a liquid ejecting head capable of ejecting a liquid and ejects a variety of liquids from the liquid ejecting head. The representative example of the liquid ejecting apparatus is an image printing apparatus, such as an ink jet printer (hereinafter, simply referred to as a printer) which includes an ink jet printing head (hereinafter, simply referred to as a printing head) serving as a liquid ejecting head and prints an image or the like by ejecting and landing liquid-like ink onto a print medium (landing target) such as a print sheet from nozzles of the printing head. In recent years, the liquid ejecting apparatus has been applied to a variety of manufacturing apparatuses such as an apparatus manufacturing a color filter such as a liquid crystal display, as well as the image printing apparatus.
A liquid ejecting apparatus is configured so as to eject a liquid from nozzles communicating with pressure generating chambers by applying an ejection driving pulse to pressure generating elements (for example, piezoelectric vibrators or heating elements), driving the pressure generating elements, changing the pressure of the liquid in the pressure generating chambers, and using this change in the pressure. In a printer disclosed in Japanese Patent No. 3412682, for example, a driving pulse (driving waveform) is used which includes an expansion step of preparing to draw a meniscus of a nozzle toward the pressure generating chamber side to the utmost, a hold step of holding this expanded state to adjust ejection time of an ink droplet, a first contraction step of ejecting the ink droplet by the contraction of the pressure generating chamber, and a second contraction step of reducing the drawing of the meniscus by reaction of the ejection operation. That is, the ink droplet is configured to be ejected by drawing the meniscus in the expansion step, contracting the pressure generating chamber, and using the reaction to the drawing of the meniscus.
However, in the foregoing known driving pulse for expanding and contracting the pressure generating chamber to eject the ink, it is difficult to eject minute liquid droplets, when a liquid (hereinafter, referred to as a high-viscosity liquid) of which a viscosity is higher than that of a liquid such as ink used in a known household ink jet printer is used.
FIGS. 8A to 8C are schematic views illustrating the movement of a meniscus of a nozzle in an operation of ejecting the high-viscosity liquid by the known driving pulse. The upper side of the drawings is the pressure generating chamber side and the lower side of the drawings is the liquid ejection side. In FIG. 8A, when the meniscus is drawn quickly toward the pressure generating chamber side in the expansion step, the central portion of the meniscus, which receives less of an influence of the inner circumferential surface of the nozzle, moves at a fast speed toward the pressure generating chamber side. However, since the portion of the meniscus closer to the inner circumferential surface of the nozzle is drawn to the inner circumferential surface of the nozzle due to the influence of the viscosity and scarcely follows the pressure change, the meniscus moves at a slow speed (hereinafter, this portion is referred to as a boundary layer). For this reason, in the known expansion step, the entire expanded meniscus cannot be drawn. In order to make the ejected ink droplet minute, it is necessary to largely draw the entire meniscus including the boundary layer, to push the central portion of the meniscus toward the liquid ejection side by reaction to the drawing, and to separate and eject only the central portion of the meniscus. However, in a case where the entire expanded meniscus cannot be drawn, the central portion of the meniscus is extruded together with the boundary layer when the central portion of the meniscus is pushed outward, as in FIG. 8B. Therefore, the ejected ink droplet becomes large.
In the high-viscosity ink, as shown in FIG. 8C, the rear portion of the ink ejected from the nozzle easily grows into a tail-like portion (drawn tail), when the ink is ejected from the nozzle. Moreover, a problem may arise in that the tail-like portion is separated from the main portion of the ink droplet and is not landed on the regular position (desirable position) of a landing target. In the ink jet printer, for example, the tail-like portion may become mist and be landed out of the regular location, so that a dot is separated. Therefore, a problem may arise in that an image quality deteriorates. In particular, in the high-viscosity liquid, since the tail-like portion is separated into several pieces, the several separated pieces (satellite ink droplets or mist) may result in deteriorating the image quality to a great extent.